Design of plate girder | Components of plate girder

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What is plate girder?

A plate girder is built-up plate and angles and connecting them by rivets or welding. The effective span for a plate girder is usually the center to center of end bearings, It is economical for heavy loads. The depth of the girder depends upon the load as well as span which is summarized below:-

S.NTypes of loadDepth of girderRemarks
1.Lighter loadsSpan/12
2.Medium loadsSpan/10
3.Heavy loadsSpan/8

Where plate girder is used?

A plate girder is used to construct the structure on which a repeatedly heavy load needs to carry. It is the modified and strong form of I section Beam. A plate girder is used where a long span is needed to carry the load. 

Components of plate girder

The different components of plate girder are used in the Design of plate girder which is given below.

Web:- Web is a vertical member of the plate girder which transfers the load of the upper flange to the lower flange.

Flange:- Flange is the horizontal member of the plate girder which transfers the direct load to the lower member of the plate girder-like web and lower flange.

Stiffener:- Stiffener is the longitudinal or horizontal member of plate girder perpendicular or parallels to the flange which is welded throughout the web connecting upper and lower flange. It reduces the shear stress on the whole plate and increases the bearing capacity of the plate girder.

Why stiffeners are used in plate girder?

Stiffeners are used in plate girders due to the following reasons,

  1. It reduces the shear stress exerted on the plate
  2. It increases the bearing capacity of the plate girder
  3. It increases the capacity to bear bending moment
  4. It restricts the web from buckling
  5. It helps to transfer tensile force on the web by the flange and makes the web strong and durable.

Advantages of plate girder?

The advantages of plate girder are:-

  • It can carry heavy load than the I section beam.
  • It provides a long span to carry the load.
  • It can be constructed by welding or riveting.
  • Different parts of the plate girder can be taken to the site separately and jointed at the working place.

Design of plate girder

 Design Assumption

The approximate design is based on the following assumption:-

  1. The shear is carried wholly by the web and shear intensity is uniform throughout the depth of the girder.
  2. The intensity of stress on flange angles and flange plates is uniform and the intensity of stress in the web varies as a maximum at the top, equal to intensity in flange plates or angle, to zero at the neutral axis.

A) Minimum thickness of plate?

S.NConditionsThickness of PlateStiffeners
1Exposed to weather but accessible for painting6 mmRarely used
2Exposed to weather but accessible for painting8 mm
3Heavy Bridge8 mm

Note:- The maximum dimension of a web panel should be maintained at 270t and the minimum dimension of the web panel should not be less than 180t, where t is the thickness of the web plate.

B) Types of web stiffener used in plate girder

Different web stiffeners are used according to different dimensions of depth and thickness of the web. Following are the different types of stiffener used in plate girders.

S.NConditionStiffeners
1If (d/Tw) = < 85No stiffener
285<= d/Tw <= 200Vertical stiffener required
3200<=d/Tw<=250Vertical stiffener and one horizontal stiffener is provided at a distance of ⅖  from the compression flange to neutral
4250<=d/Tw<=400Plus one horizontal stiffener at neutral
5At support and concentrated loadBearing stiffeners are provided under concentrated load so as to avoid local bending of the flange as well as local buckling of the web.

C) Self-weight of plate girder

  1. For riveted plate girder = W/300 per meter run
  2. For welded plate girder = W/400 per meter run

Where, W = Total superimposed load on the girder

Where, 

D = Depth in mm

M = Bending moment in N-mm

f  = Permissible stress in Mpa

t = Thickness of plate in mm

Some important points to remember for Design of plate girder

  1. The minimum spacing of vertical stiffeners in plate girder should not be less than 0.33 d.
  2. The maximum spacing of vertical stiffener in plate girder should not be less than 1.50 d.
  3. The effective length of bearing stiffener can be taken 0.70 times of actual length.
  4. The least permissible clear dimension of the web in th panel of a plate girder should not be less than 10t.
  5. The greatest permissible clear dimension of the web in the panel of a plate girder can be taken as 270t.
  6. The length of outstanding leg of angle stiffeners is limited to 16t, where t is the thickness of angle.
  7. The length of outstanding leg of flat stiffeners is limited to 16 t, where t is thickness of flat.
  8. More than 90% shear is taken by the web of girder.

I hope this article on “Design of plate girder | Components of plate girder” remains helpful for you.

Happy Learning – Civil Concept

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